1. Field of the Invention
The present invention relates generally to improvements in digital telephones and, more particularly, pertains to new and improved gain imbalance compensation for quadrature receivers in cordless direct sequence spread spectrum telephones.
2. Description of Related Art
The cordless telephone products that exist today for the most part use analog frequency modulation in the 46-49 MHz band, and provide good speech quality when the handset is at close proximity to the base station. Although these analog cordless telephones are inexpensive and offer advantages to the customer which have made the products a successful replacement for corded telephones, these analog phones still leave much room for improvement. For example, it would be desirable to have a longer handset-to-base station range without dropouts, more channels, and greater security.
Use of digital modulation and digital coding techniques offers more robust voice communication over a radio channel, although requiring greater channel bandwidth. Digital modulation also has a capture effect that greatly surpasses co-channel and adjacent channel interference, thereby providing a more noise-free conversation. Use of digital modulation encoding also allows for the addition of effective scrambling codes to thereby greatly improve telephone security. In addition, by utilizing the industrial, medical, and scientific (ISM) band for radio transmission and reception (902-920 MHz), increased power levels above those in the 46-49-MHz band are permitted, thus increasing the operating range. A requirement for highest power in the ISM band is that direct sequence spread spectrum (DSSS) or FHSS modulation be utilized.
Gain imbalance exists on radio systems that are quadrature receivers mixed to basebands with analog techniques. Radio systems generally have gain imbalance between I and Q receiver paths due to the difficulty in matching gains in the in-phase and quadrature paths. This gain imbalance offsets the accuracy of demodulation and consequently degrades the performance of the quadrature receiver. Typical direct sequence spread spectrum systems are incapable of digitally correcting for the errors caused by the gain imbalance because of the low A/D dynamic ranges used.